As shown in FIG. 18, a conventional motor speed control device for controlling a motor speed of a capstan motor or a drum motor of a magnetic recording-reproducing device such as a VCR is provided with, for example, a rotation detecting sensor (referred to as FG (Frequency Generator) sensor hereinafter) 52, such as an MR (Magnetic Resistance) sensor, for detecting an magnetic pattern magnetized on a rotating section of a motor 51, a waveform shaping circuit 53 for amplifying an output signal of the FG sensor 52 so as to output an FG pulse signal proportional to the motor rotation, a period calculating unit 54 for outputting period information of the FG pulse signal, an adder 55, an amplifier 56 for amplifying and phase compensating an error signal outputted from the adder 55 so as to output a speed commanding value, and a motor driving circuit 57 for driving the motor 51 according to the speed commanding value.
In this motor speed control device, period information (rotation information of the motor 51) is compared with a target value, and the difference between the period information and the target value is applied to the motor 51 so as to feedback-control the rotation speed of the motor 51 in such a manner that the period information approaches the target value. Here, in order to control the motor 51 more accurately, the amplification degree, or a servo gain is increased by changing the phase compensation value of the amplifier 56.
However, in the feedback-control of the conventional motor speed control device, because the servo gain of the amplifier 56 cannot be increased to infinity, there is a limit in controlling the motor speed with high accuracy.
Namely, as parameters for judging whether the control system is stable, for example, as shown in the Bode diagram of FIG. 19, a phase margin and a gain margin are available. The phase margin is a phase difference between the phase when the gain characteristic is 0 dB and the phase of -180.degree., and the gain margin is a gain when the phase characteristic is -180.degree..
Generally, a phase margin of 40.degree., and a gain margin in a range of -10 dB to -20 dB are preferable. When the phase margin and the gain margin decrease, a stable control cannot be achieved.
In the Bode diagram, when the servo gain is increased, the gain characteristic is shifted in an upward direction so that the phase margin and the gain margin decrease. Thus, the servo gain is determined by the phase characteristic. For this reason, in order to maintain a stable control system even when the servo gain is increased, a phase delay is shifted to the side of the high frequency band. Namely, the control band is widened.
The factors determining the control band are 1 the detecting period of the motor speed (period of the FG pulse signal), 2 phase compensation by the amplifier 56, 3 data transfer time and calculation time of A/D and D/A conversions etc. in the case where a series of processes are to be carried out by a computer software.
As for the factor 3, the phase delay can be suppressed to some degree with the aid of a high speed microcomputer or high speed IC. As for the factor 2, the phase delay can be suppressed to some degree if the phase compensation is optimally set by a designer considering the calculation time and the gain characteristic of a phase compensator.
As for the factor 1, the phase delay can be decreased when the detecting period of the motor speed is made shorter. This can be achieved by increasing the number of output pulse counts of the FG sensor 52 in one rotation of the motor. However, there is a limit to this due to the positional relationship between the magnetization pitch and the FG sensor 52.
Namely, in order to read an S/N pattern on a narrow magnetization pitch, it is required that the FG sensor is in a vicinity of a magnetization surface. However, there is a limit to this due to the oscillation accuracy etc. of the magnetization surface.
Further, due to the recent miniaturization of the motor speed control device as well as the motor 51, it is difficult to increase the number of magnetization, e.g., the number of magnetic poles, and when the motor is rotating at a low speed, the FG pulse signal period becomes even longer.
As a countermeasure, as a method for shortening the FG pulse signal period, the following methods are available. As shown in FIG. 2, a method for obtaining a detecting period twice the number of the pulse period by using a rising edge-falling edge period t(0) and a falling edge-rising edge period t(1) of the FG pulse signal, and as shown in FIG. 9, a method for obtaining a detecting period (2.times.m) times the number of the pulse period by using respective periods t(0), . . . , t(2.times.m -1) between each edge wherein FG sensors are provided in m quantities (m.gtoreq.2 where m is an integer) so as to be out of phase with each other.
However, in the described methods in which two types of periods are obtained, it is difficult to obtain accurate period information due to a duty error in which a difference is generated in each period by a threshold level deviation etc. of the waveform shaping circuit 53, and a phase error generated by a mounting phase deviation of the FG sensor 52. Also, when the motor 51 is to be controlled by such information having periodic nonuniformity, problems such as a motor speed fluctuation destabilizing the motor speed and a motor noise are presented.